In the oil and gas production industry wellbores are drilled into the earth to intercept subterranean hydrocarbon bearing formations and then lined with metal tubulars and cement to provide sealing and support. Once a bore has reached its required depth and is suitably lined, production or completion architecture is deployed into the bore to facilitate controlled production of hydrocarbons. Typical completion architecture includes tubing strings, for example production tubing strings which provide a continuous flow path between the subterranean formation and the surface. Production tubing strings may also carry appropriate tools and other completion equipment, such as valves, packers, sensor suites and the like.
Tubing strings are generally formed and deployed in sections, with individual sections secured to each other by threaded connectors and deployed in a staged manner. To enable deployment it is required to permit the tubing string to fill with the wellbore fluid, typically called a completion fluid. It is also desirable to be able to test the pressure integrity of the tubing string, particularly the integrity of the threaded connectors, during the process of deployment.
Arrangements are known in the art which provide an open orifice in the lower end of a tubing string which permits filling during deployment. Whenever pressure testing is required a plug may be run from surface, for example on coiled tubing, to block the orifice and permit the internal tubing pressure to be increased for testing purposes. Following this the plug may be retrieved to surface. However, such an arrangement requires repeated deployment and retrieval of additional equipment to/from the wellbore, increasing deployment time and the risk of equipment becoming stuck in the bore.
Other arrangements are known, for example from U.S. Pat. No. 7,063,156, which are based on the use of a flapper valve mounted at the lower end of a tubing string. During deployment the relative motion of the tubing string and wellbore fluid causes the flapper valve to open, usually against the bias of a spring, to permit filling of the string. When pressure testing is required deployment of the string is temporarily arrested permitting the flapper to close, for example under the action of a bias spring, allowing the valve to support pressure from above to facilitate pressure testing. Once the tubing string is fully deployed, the flapper valve may be actuated, for example by an internal sleeve, to be permanently held open, thus providing a permanent flow path through the string. In such prior art arrangements, pressure for testing the string is retained by the flapper valve which also functions to permit filling of the string. Accordingly, the flapper valve and associated sealing structures are at risk of damage and clogging by debris carried within the wellbore fluid entering the string, which can adversely affect the sealing integrity of the flapper valve, reducing its ability to support desired pressure testing.